CBS summer perennial “Big Brother” opened its 15th season Wednesday down from last year, enabling Fox’s rising “MasterChef” to remain the night’s top draw in key demos.

Nielsen estimates that “Big Brother” averaged a 2.1 rating/7 share in adults 18-49 and 6.3 million viewers overall in the 8 o’clock hour, on par with Fox’s “MasterChef” in demos and leading the timeslot in total viewers. But “Big Brother” was down from its mid-July season premiere of last year (2.6/8 in 18-49, 7.2 million viewers overall), which aired at 9 p.m. after a highly rated repeat of “The Big Bang Theory.”

Compared to its first Wednesday episode of last year (2.0/7 in 18-49, 5.8 million viewers overall), “Big Brother” was up a little. The show will again air three times this summer, including twice in a 24-hour period over Tuesday and Wednesday. This is the first time “Big Brother” has premiered in June, and this season — at nearly three months in length — will be the show’s longest to date.

Following “Big Brother,” the timeslot premiere of “The American Baking Competition” (1.2/4 in 18-49, 5.1 million viewers overall) was on par with its performance in the 8 o’clock hour last week. And at 10, a special repeat of “Criminal Minds” led the broadcast networks with a 1.2/4 in 18-49 and 5.6 million viewers overall.

Fox won the night in demos with back-to-back hours of “MasterChef” at 8 (2.1/7 in 18-49, 5.4 million viewers overall) and 9 (2.4/8 in 18-49, 6.1 million viewers overall), both of which were up a tick from last week. The 9 p.m. installment of “MasterChef” was Wednesday’s No. 1 broadcast in 18-49 and 25-54 (2.9/8).

NBC aired an original “Dateline” (1.2/4 in 18-49, 5.5 million viewers overall), which was up slightly vs. its most recent Wednesday original, and encores of dramas “Law & Order: SVU” (1.1/3 in 18-49, 5.0 million viewers overall) and “Chicago Fire” (0.7/2 in 18-49, 2.9 million viewers overall).

And ABC was led by a repeat “Modern Family” (1.1/3 in 18-49, 3.6 million viewers overall), the finale of “How to Live With Your Parents” (0.9/3 in 18-49, 3.0 million viewers overall) and new series “The Lookout” (0.9/3 in 18-49, 3.5 million viewers overall), which was down from the 1.1 demo rating it had done the last few weeks. Opening the night for the Alphabet were a repeat “The Middle” (0.8/3 in 18-49, 3.6 million viewers overall) and a new “Family Tools” (0.7/2 in 18-49, 2.9 million viewers overall), which hit a low as it nears the end of its run.